The invention relates to a process of the type for producing on a continuous basis boards made of wood chips or other particles, in which the particles are bonded together and hardened under the application of pressure and heat, as well as to a corresponding apparatus, as are known from the German Patent 23 55 797.
An apparatus of this type represents a considerable investment and, therefore, the operators have the desire not only to be able to produce boards with widths corresponding to the nominal working width on such an apparatus, but also, if needed, narrower boards. For example, boards widths of 210 and 185 cm are conventional on the market. Problems occurred previously when the attempt was made to run the narrower board on an apparatus designed for the wider board by adjusting the width of the filling to be correspondingly narrower, because the edges of the forming belts projecting beyond the edge of the filling were no longer subjected to a counter-pressure and were no longer adequately pressed against the support structure, from which not only the pressure, but also the heat, is transferred to the forming belts. Therefore, the forming belts no longer had any thermal contact at the edge with the support structure or with the rollers in the case of the construction disclosed in the German Patent 23 55 797, which rollers transmit the heat from the support structure to the forming belts and roll over the entire width of the forming belts, so that the temperature of the forming belts dropped significantly toward the edge. As a result, the edge zones shrank lengthwise and considerable thermal stresses developed since the wide middle zone of the forming belts was at working temperature. Such thermal stresses became critical in the area of the reversing drums because there the thermal stresses combined with the stresses being added by the significant longitudinal tension of the forming belts and by the elongation of the outer fibers due to the rerouting of the belts. As a result, total tensile stresses developed on the outside of the areas of the forming belts guided over the reversing drums. These stresses approached, and sometimes exceeded, the yield stress. In any case, however, they led to problems during continuous operation, especially as the forming belts are made of stainless steel, which is not particularly resistant to lasting bending strains.
Similar problems already occurred even earlier in double-belt presses, and in fact even when running with the nominal width. The filling, namely, does not extend exactly up to the edge of the forming belts. Rather, these forming belts projected transversely by a certain amount beyond the filling and also beyond the edge of the zone covered by the rollers. Here again, there were temperature drops and associated thermal stresses.
In the case of the press according to the German Patent 22 43 465, an attempt was made to keep the temperature drop within limits by heating the projecting edges of the forming belts. However, it was found to be necessary to heat the edges of the forming belts over practically their entire length, because otherwise the temperature immediately dropped off outside of a heated spot. However, heating the entire length of the forming belts entails significant structural problems and generally does not come under consideration due to the considerable expense.
According to the German Patent 28 19 943, another solution is found by corrugating the projecting edge of the forming belts, so that in case of a temperature drop toward the edge, more material is available there to an extent, and so that the longitudinal tensile stresses which occur in case of a thermal contraction are not so great. To be sure, this measure is practicable for edges projecting by a few centimeters, however no longer when these edges, in which a temperature drop is to be noted, amount to several tens of centimeters.
A solution to the problem is known from the German Published Patent Application 37 04 940, in which an edge filling of unbonded particles is spread on to the bottom forming belt and pressed with it in the edge zone extending outside of at least one edge of the filling producing the boards up to close to the edge there of the compression section. To be sure, excellent results are obtained with this method. However, the expenditure for additional spreading equipment and the subsequently added return systems for the spread, unbonded particles at the end of the compression section is considerable.
The object of the invention is to develop this type of process and such an apparatus so that a contact arrangement of the forming belts at the roller chains which ensures adequate thermal transfer to the forming belts at the edge can be achieved with minimal expenditure for equipment.